Files of the type having a large number of light-pervious plastic film-made pockets bound together are known. Such plastic film pockets are generally made available as synthetic resin-made pockets having good transparency in order that they may be effectively used for storing and preserving various reference materials, documents and valuable collections. A support sheet is inserted in such a plastic film pocket for purposes of shape retention and otherwise.
A method of making such plastic film pockets is known such that, for example, a synthetic resin, such as a polyethylene resin, is continuously molded by inflation molding into continuous thin-film tubular material having relatively high transparency, which in turn is heat sealed at portions to be used as lower ends of pockets and is cut at each such portion. Cutting is made to a desired size for plastic film pockets so as to define a portion which serves as the upper end of each pocket.
The insertion of support sheets into plastic film pockets is effected in such a manner that after the upper end of each pocket is opened, two support sheets are inserted through the opening into the pocket to assume laterally spaced positions therein. Files are fabricated by placing a plurality of support sheet-containing plastic film pockets obtained in manner as above noted in superposed relation on a sheet material which serves as an outer cover sheet, and welding them together at central portions thereof in which no support-sheet is present.
According to such known method of making plastic film pockets, however, it is necessary that support sheets must be inserted into each pocket which is already made in the form of a pouch. This involves a difficulty in that it is impracticable to mechanize the step of support sheet insertion or carry out the step without manual operation. Indeed, considerable labor is required for proper insertion of two support sheets into position.
As such, the supply of support sheet-containing plastic film pockets does not catch up with growing demand for files using such film pockets, and this has been the largest factor for low productivity in the manufacture of files.